Nonlinear optical fibre method of its production and use thereof

ABSTRACT

An optical fiber having a longitudinal direction and a cross-section perpendicular thereto, said fiber in a cross-section comprising: (a) a core region ( 11 ) having a refractive index profile with a highest refractive index n c , and (b) a cladding region comprising cladding features ( 10 ) having a center-to-center spacing, Λ, and a diameter, d, of around 0.4Λ or larger, wherein n c , Λ and d are adapted such that the fiber exhibits zero dispersion wavelength of a fundamental mode in the wavelength range from 1530 nm to 1640 nm; a method of producing such a fiber; and use of such an optical fiber in e.g. an optical communication system, in an optical fiber laser, in an optical fiber amplifier, in an optical fiber Raman amplifier, in a dispersion compensator, in a dispersion and/or dispersion slope compensator.

This is a nationalization of PCT/DK03/00178 filed Mar. 14, 2003 andpublished in English.

1. BACKGROUND OF THE INVENTION

The present invention relates to a nonlinear optical fibre having asmall core and special dispersion properties, a method of producing sucha fibre, and use of such a fibre.

THE TECHNICAL FIELD

Nonlinear optical fibres with zero-dispersion wavelength (ZDW) around1.5 μm are attractive for a range of telecom applications such as 2Rregeneration, multiple clock recovery, optical parametric amplifiers(OPAs), pulse compression, soliton generation, wavelength conversion,alloptical switching and supercontinuum-based wavelength demultiplexed(WDM) telecom sources (see for example Petropoulos et al., Optical FiberCommunication Conference, 2001; Futami et al., European Conference onOptical Communications, 2000; Hansryd et al., Photonics TechnologyLetters 13(3), pp. 194-196, 2001; Sharping et al., IEEE PhotonicsTechnology Letters, Vol. 14 Issue. 1, pp. 7-79, 2002; or Takushima etal., Photonics Technology Letters 11(3), pp. 322-324, 1999).

In the prior art, nonlinear optical fibres with ZDW around 1.5 μm havebeen based on standard optical fibre technology, where a solidhigh-index core is surrounded by a solid cladding having a lowerrefractive index than the core (see for example U.S. Pat. No.6,347,174). Typically prior art nonlinear fibres with ZDW around 1.5 μmhave a nonlinear coefficient of approximately 10 (Wkm)⁻¹. Forapplications such as e.g. OPAs, nonlinear coefficients of this orderdemands fibre lengths on the order of hundreds of meters.

Within the past five years, a new type of optical fibre has emerged.This type of fibre employs microstructured features running along thefibre axis, and is referred to by several names, e.g. microstructuredfibres, photonic crystal fibres (PCF), holey fibres, or hole-assistedfibres.

Typically the microstructured features are voids (such as for exampleair holes) and the voids may provide this fibre type several advantagescompared to standard technology nonlinear fibres. The high indexdifference between the silica core and air-filled microstructurefeatures enables tight mode confinement to the fibre core—therebyresulting in a small effective area and consequently a high nonlinearcoefficient. In addition, the ability of the cladding structure to betailored enables high flexibility in the design of the dispersionprofile (see for example Ferrando et al., Optics Express 9(13), pp.687-697, 2001; Monro et al. Journal of Lightwave Technology, Vol. 17,No. 6, pp. 1093-1102, 1999; or WO 0212931). This flexibility facilitatesgeneration of different nonlinear effects, especially by the choice ofzero-dispersion wavelength (ZDW).

In particular, microstructured fibres may be realized with very smallcores (smaller than 3 μm in diameter) and ZDW at relatively shortwavelengths (shorter than 1.3 μm). Such nonlinear microstructured fibreshave received a significant interest (see for example U.S. Pat. No.6,097,870) and a much studied area of these fibres is super continuumgeneration with pumping wavelength around 600-800 nm (see for exampleRanka et al., Optics Letters 25[1], pp. 25-27, 2000; Coen et al. OpticsLetters 26(17), pp. 1356-1358, 2001., or Hansen et al., LEOS Conference2001, 2001. A number of potential applications exist of super continualincluding optical coherence tomography, spectroscopy, and meteorology.

PRIOR ART DISCLOSURES

While nonlinear microstructured fibres with ZDW shorter than 1.3 μm havebeen extensively studied, prior art microstructured fibres with ZDWaround 1.5 μm have only been addressed in theoretical works—see forexample Ferrando et al., Optics Express 9(13), pp. 687-697, 2001; Monroet al. Journal of Lightwave Technology, Vol. 17, No. 6, pp. 1093-1102,1999; or WO 02/12931. The fibres disclosed by Ferrando et al. and Monroet al. with ZDW around 1.5 μm have a microstructured cladding comprisingclose-packed air holes with a diameter, d, of around 0.3 times acentre-to-centre spacing, Λ. The fibres are further characterized by asolid, pure silica core with a diameter of around 3.9 μm or larger (thecore diameter being approximately equal to 2Λ-d).

White et al. in Optics Letters 26(21), pp. 1660-1662, 2001 disclose amicrostructured fibres with large air holes in order to reduceconfinement losses. Nothing is indicated nor suggested about tailoringof dispersion properties, in particular to provide ZDW around 1.5 μm.Further nothing is disclosed about a flat dispersion slope.

Barkou et al., WO 02/12931, disclose a nonlinear microstructured opticalfibre having an inner cladding and an outer cladding wherein the innercladding holes are smaller than the outer cladding holes. Nothing isindicated nor suggested about providing an optical fibre with anon-partitioned cladding with substantially equal sized claddingfeatures.

Generally, it is desired to provide ZDW or near-zero dispersion over anextended wavelength range. Hence, it is generally desired to be able tocontrol not only the dispersion at a given wavelength, but also thedispersion slope of nonlinear optical fibre. The dispersion slope is thederivative of the dispersion with respect to wavelength and, thereby,control of the dispersion slope provides control of the dispersion overa broad wavelength range. This facilitates broadband applications, forexample continuum generation over broad wavelength ranges and nonlinearprocesses, where a pump and signal light are broadly separated.Depending on the specific application, the broadness in terms ofwavelength range may be a few nanometres or it may be several hundredsand even more than a thousand nanometres.

Another important area of applications of nonlinear optical fibresincludes Raman amplification (see for example Agrawal, “Nonlinear fiberoptics”, Third Ed., Academic Press; Yusoff et al. Optics Letters, Vol.27, No. 6, 2002). In optical communications, Raman fibre amplifiers areuse for both discrete amplification and distributed amplification. Inthe former case the nonlinear fibre is inserted as an component at oneor more discrete site of an optical communication link, whereas in thelatter case the nonlinear fibre constitute a part of the transmissionlink in an optical communication system. Especially, for use ofnonlinear optical fibre in optical communications, it is important tocontrol the dispersion and often also the dispersion slope of thenonlinear optical fibre. Hence, in order to control for the developmentof nonlinear fibres for optical communication systems, there are threeimportant parameters: the nonlinear coefficient, the dispersion, and thedispersion slope. An important use of nonlinear optical fibre in opticalcommunication systems is as Raman amplifier that at the same timeprovides dispersion and/or dispersion slope compensation (see forexample Gnauck et al. Optical Fiber Communication Conference, postdeadline paper FC-2, 2002, where dispersion compensating modules arealso used for Raman amplification). Hence, it is desired to providenonlinear fibres with dispersion and/or dispersion slope compensatingcharacteristics that at the same time has a high nonlinear coefficientin order to allow for Raman amplification.

2. DISCLOSURE OF THE INVENTION

Object of the Invention

It is an object of the present invention to provide new fibre designs ofnonlinear fibres, where the nonlinear coefficient, the dispersion andthe dispersion slope may be tailored more freely than for prior artnonlinear fibres.

It is a further object of the present invention to provide new designsof nonlinear optical fibres, where the dispersion and the dispersionslope may be tailored precisely, while a high nonlinear coefficient issimultaneously obtained.

It is, therefore, an object of the present invention to providenonlinear microstructured optical fibres with ZDW around 1.5 μm for holesizes larger than d/Λ=0.3.

Further objects appear from the description elsewhere.

Solution According to the Invention

In a first aspect, the present invention relates to an optical fibrehaving a longitudinal direction and a cross-section perpendicularthereto, said fibre in a cross-section comprising:

-   (a) a core region having a refractive index profile with a highest    refractive index n_(c), and-   (b) a cladding region comprising cladding features having a    centre-to-centre spacing, Λ, and a diameter, d, of around 0.4Λ or    larger,-   wherein n_(c), Λ and d are adapted such that the fibre exhibits zero    dispersion wavelength of a fundamental mode in the wavelength range    from 1530 nm to 1640 nm;-   whereby fibre designs with a relatively high nonlinear coefficient    can be obtained.

PREFERRED EMBODIMENTS

In a preferred embodiment, the microstructured optical fibre comprises aglass background material in the cladding and/or in the core, such as asilica-based glass or a compound glass. Glass materials are preferred inorder to a provide low-loss background material of the fibres, as wellas a material that may be handled using presently known techniques forfabricating microstructured optical fibres.

In a further preferred embodiment, the cladding features are voids, suchas holes filled with air, another gas, or a vacuum. Such features arepreferred in order to utilize a high refractive index contrast to thefibre background material, as this provides flexibility for tailoringdispersion characteristics of the fibre as well as provide smalleffective mode field diameters.

In a further preferred embodiment, the microstructured optical fibrecomprises a background material in the cladding with a refractive indexin the range from 1.43 to 1.47, such as around 1.444. Such refractiveindex ranges are feasible using silica based materials, including pure,fused silica and silica doped with various materials, such as forexample Ge, F, Al, P, Sn, B, La or combinations of these.

In a further preferred embodiment, n_(c) is in the range of around 1.444to 1.49, such as around 1.47. The core regions is preferred to have ahigher refractive index than the background material of the cladding,and low-loss glass may be fabricated using silica technology withrefractive indices of up to at least 1.49.

Compound glasses (such as for example chalcogonide glasses) may bepreferred due to higher intrinsic nonlinearindex coefficient thansilica-based glasses. Furthermore, for compound glasses the refractiveindex may be varied more freely than for silica glasses and therefore inpreferred embodiments, the nonlinear microstructured optical fibrecomprises compound glasses.

In a further preferred embodiment, the microstructured optical fibrecomprises at least five rings or periods of cladding featuressurrounding the core, such as at least six rings or periods of claddingfeatures surrounding the core, such as at least seven rings or periodsof cladding features surrounding the core in order to reduce leakagelosses and provide low loss transmission in the fibre.

In a further preferred embodiment, a majority of the cladding featuresbeing substantially equal in size. During fabrication it is oftenpreferred to have voids of similar holes, and in order to provide ZDWaround 1.5 mm, it is preferred that at least the three innermost ringsof voids are substantially equal in size. Preferably, more ringssurrounding the core have equally sized holes, such as at least four,five or more rings. A person skilled in the art will recognise thatminor differences in size may results from the fabrication process.Typically such variations are less than a few percent.

In a further preferred embodiment, the microstructured optical fibrecomprises cladding features with diameters of around 0.8 μm, such as inthe range from 0.7 μm to 0.9 μm.

In a further preferred embodiment, the microstructured optical fibrecomprises cladding features with centre-to-centre spacing, Λ, of around1.6 μm, such as in the range from 1.2 μm to 1.8 μm.

In a further preferred embodiment, the core region has a core diameter,d_(c), being defined as 2Λ-d, and this core diameter is in the range of1.5 μm to 3.0 μm. In particular it is preferred that the core diameteris in the range of 2.2 μm to 2.5 μm.

In a further preferred embodiment, the core region comprises a corefeature having a diameter, d_(c,feat), being smaller than or equal tod_(c). The core feature takes part in defining the refractive indexprofile of the core region, and the maximum refractive index of the corefeature is identical to the maximum refractive index of the core region.

In a further preferred embodiment, the core feature has a diameter,d_(c,feat), in the range of 0.2 d_(c) to 0.9 d_(c), such as in the rangeof 0.4 d_(c) to 0.7 d_(c), such as in the range of 0.45 d_(c) to 0.55d_(c), such as around 0.50 d_(c).

Other aspects and preferred embodiments appear elsewhere in theapplication.

It is to be understood that the following detailed description andaccompanying drawings further describe principles and operation ofexemplary embodiments of the invention, and that the invention is notintended to be limited thereto.

3. BRIEF DESCRIPTION OF THE DRAWINGS

In the following, by way of examples only, the invention is furtherdisclosed with detailed description of preferred embodiments. Referenceis made to the drawings in which

FIG. 1 shows a schematic example of a preferred embodiment of fibreaccording to the present invention;

FIG. 2 shows microscope pictures of a preferred embodiment of anonlinear microstructured optical fibre according to the presentinvention;

FIG. 3 shows dispersion properties of the fibre in FIG. 2 that has ZDWaround 1.56 μm;

FIG. 4 shows the spectral attenuation of the fibre in FIG. 2 whenspooled to a radius of 9 cm (solid line indicates white lightmeasurements and the two points OTDR loss measurements. Inset shows OTDRtrace at 1310 nm);

FIG. 5 shows a schematic example of another preferred embodiment of afibre according to the present invention;

FIG. 6 shows a schematic example of a preferred embodiment of apolarization maintaining fibre according to the present invention;

FIG. 7 shows dispersion and effective area properties of a fibreaccording to the present invention with flat, near-zero dispersion overabout 100 nm around a wavelength of 1.55 μm;

FIG. 8 shows the mode field distribution of the fibre of FIG. 7;

FIG. 9 shows dispersion properties of a series of embodiments of fibresaccording to the present invention with variation of various designparameters;

FIG. 10 shows effective area properties of a series of embodiments offibres according to the present invention with variation of variousdesign parameters;

FIG. 11 shows dispersion properties of another series of embodiments offibres according to the present invention with variation of variousdesign parameters;

FIG. 12 shows dispersion properties of a series of embodiments of fibresaccording to the present invention with variation of a single designparameter;

FIG. 13 shows dispersion and effective area properties of two differentembodiments of fibres according to the present invention with flat,near-zero dispersion over about 100 nm around a wavelength of 1.55 μm;

FIG. 14 shows dispersion properties of two different embodiments offibres with ZDW around 1064 nm;

FIG. 15 shows a schematic example of a preform for producing a fibreaccording to the present invention; and

FIG. 16 shows a schematic example of a preform cane for producing afibre according to the present invention.

4. DETAILED DESCRIPTION

FIG. 1 shows a schematic example of an embodiment of a fibre accordingto the present invention comprising a microstructured cladding withclose-packed features 10 having a diameter, d, and a centre-to-centrespacing, Λ. The fibre comprises further a core with a high index feature11 having a higher refractive index than the background material of thecladding. The fibre further comprises a solid overcladding 12. The solidovercladding mainly serves to provide mechanical stability of the fibreand a given desired outer fibre diameter. Typically, another embodimentof a fibre according to the present invention includes further acoating—typically a polymer coating—for further mechanical stability ofthe fibre.

The fibre in FIG. 1 has d of around 0.8 μm and Λ of around 1.6 μm, andthe number of periods or rings of cladding features surrounding the coreis six, but higher numbers may be preferred in order to lower leakage orconfinement losses. The fibre core comprises a feature of maximumrefractive index, n_(c), being around 2% higher than the refractiveindex of the cladding background material.

FIG. 2 shows microscope pictures of a fabricated fibre of an embodimentaccording to the present invention. The fibre has a solid silica coreregion with a diameter of around 2.3 μm (the core diameter defined forthe given design as 2Λ-d). The core region comprises a high-indexfeature with a diameter of around 0.8 μm in diameter. The high-indexfeature, not visible in the pictures, has a parabolic index profile, buta broad range of other profiles are also covered by the presentinvention, and the maximum refractive index, n_(c), of the core featureis around 2% higher than the refractive index of pure silica.

It turns out that the high-index feature in the core region providesincreased nonlinear refractive index of the core, as well as it providesa smaller mode field diameter, and furthermore it aids to reduceconfinement loss. The microstructured cladding consists of six periodsof holes in a close-packed (also known as triangular) structure with anaverage hole-size of approximately 0.8 μm and an average orrepresentative centre-to-centre spacing, Λ (or average pitch) ofapproximately 1.6 μm.

Embodiments of optical fibre according to the present invention may beobtained using methods known in the art, such as described by DiGiovanniet al. in U.S. Pat. No. 5,802,236, or Broderick et al. in WO 02/14946.Embodiments of the present fibre were produced in a step-wise process,where capillary tubes of approximately 2 mm in diameter were preparedand arranged in a periodic structure. A single, central capillary tubewas replaced by a solid cane comprising a high-index region. Thestructure was placed in an overcladding tube to provide a preform andthe preform was drawn to a cane with a diameter of around 3.6 mm using aconventional drawing tower operating at a temperature of around 1900° C.The cane was afterwards placed in a second overcladding tube and drawninto a second cane that again was overcladded and finally drawn to fibreusing the same conventional drawing tower. The preform may be preparedby controlled heat treatment, optionally under pressure and/or vacuum ofthe capillary tubes and the interstitial voids between the tubes. Askilled person would know how to calibrate the parameters of thepreparation, e.g. the temperature, pressure, vacuum, with respect to theglass of the capillaries applied, e.g. its thickness, viscosity,softness, etc., see e.g. the afore-mentioned references by DiGiovanni etal. or Broderick et al., the contents of which are incorporated hereinby reference.

The fabricated fibre shown in FIG. 2 had a microstructured region beingslightly elliptical (2.8%), resulting in an ellipticity of the coreregion. The ellipticity gives rise to a relatively strong birefringencethat makes the fibre polarization maintaining. The birefringence hasbeen measured using a fixed analyzer technique yielding a birefringence,Δn of approximately 1.1·10⁻⁴ at 1550 nm. This degree of birefringence isequivalent to a mean differential group delay of 0.37 ps/m or a beatlength of 14 mm.

Despite a small core size and an air-filled microstructured region,embodiments of fibres according to the present invention may be splicedto standard optical fibres using commonly available splicing equipment.Using a commercially available optical fibre with an ultra high NA, thefibre in FIG. 2 was spliced with a loss of around 0.3 dB.

In order to estimate the nonlinear coefficient of the fibre in FIG. 2,the near field has been measured at 1550 nm yielding an effective areaof approximately 7 μm². Consequently, the fibre is estimated to have anonlinear coefficient of at least 20 (Wkm)⁻¹, which is at leastcomparable to highly nonlinear fibres. Key data for the microstructuredoptical fibre is listed in table 1.

Average pitch:  1.6 μm Average hole diameter:  0.8 μm Core diameter: 2.3 μm Fibre diameter: 126 μm Splice loss:  0.3 dB Numerical aperture:~0.5 Birefringence: 1.1 · 10⁻⁴ Nonlinear coefficient: ~20 (Wkm)⁻¹

The uniformity of the fibre structure in FIG. 2 has been investigated bymeasuring pitch and core size along the three symmetry axis at fourequally spaced positions in the fibre. The variation in pitch along thefibre is 2.4% and variation in core size is <1.5%. The data is shown intable 2.

Position Average pitch Average core size [m] [μm] [μm] 0 1.62 2.3 501.59 2.3 100 1.62 2.3 150 1.63 2.4

FIG. 3 shows four measurements of the dispersion of the fibre in FIG. 2:a measurement on a 150 m master spool and three measurements on therewinds in which the master spool has been divided for uniformity test.The measurements show good uniformity along the fibre as the ZDW ispositioned within a 6 nm band for the three parts of the fibre,equivalent to a 0.4% variation. It is noted that the 2.4% structuralnon-uniformity only results in a 0.4% variation in ZDW indicatinginherent robustness in the design. The dispersion slope is in the range−0.25 to −0.27 ps/km/nm² at the ZDW. The negative slope of the fibre isespecially interesting in combination with standard nonlinear fibreswith positive dispersion slope, which enables creation of nonlineardevices with near-zero slope and low dispersion in a large wavelengthrange.

FIG. 4 shows measurements of the spectral attenuation in the fibre ofFIG. 2. The figure shows a low loss below 0.03 dB/m in the range from700 nm to 1300 nm and a steep loss edge starting around 1500 nm. Theposition and slope of this loss edge is unaffected by spool radius andthe present inventors have realized that the increase in loss around1500 nm arises from increasing confinement losses. Loss measurementsperformed with OTDR at 1310 nm and 1550 nm yield losses of 0.017 dB/mand 0.058 dB/m respectively, which is consistent with the white-lightmeasurements. To improve the fibre of FIG. 2, more than five, such asmore than six, such as more than seven periods of voids surrounding thecore may be preferred. Apart from the confinement loss edge, the loss inthe fibre is dominated by impurities and Si—OH absorption. Such lossesmay be lowered by improved quality of the material for the preform, byimproved cleanness during preform assembling and handling, as well asduring fibre drawing.

Another embodiment of a fibre according to the present invention isshown in FIG. 5. The fibre has an improved design to allow for a reducedispersion slope characteristics, i.e. the fibre may exhibit a zero ornear zero dispersion over a broad range of wavelengths. The reducedslope is obtained by raising the index around the core to a level higherthan that for a fully periodic triangular structure with solid core. Incontrast to the designs disclosed by Barkou et al. in WO 02/12931, thereduced slope is obtained by comprising—within a cross-section of thefibre—features or elements of a refractive index being lower than thecore and preferably lower than the cladding background material, buthigher than the material of the cladding holes. Alternatively, thedesign may be seen as being realized by a number of air holes around thecore being replaced with down-doped glass features or elements, areas.Hence, FIG. 5 shows schematically a cross-section of an embodiment of afibre 50 according to the present invention comprising a core 51 and amicro-structured cladding. The core comprises a material with refractiveindex n_(c), and the cladding comprises a number of low-index claddingfeatures 52 with refractive index n₁ that are placed in a claddingbackground material 53 with refractive index n_(b). Immediatelysurrounding the core 51 are placed one or more first inner claddingfeatures 54 and one or more second inner cladding features 55. The firstinner cladding features have a refractive index similar to the low-indexcladding features, namely n₁. The second inner cladding secondaryfeatures have a refractive index n₂. The fibre is characterized in thatn₂ is lower than n_(c) and n₂ is higher than n₁.

Alternatively, the fibre in FIG. 5 may be seen as an improvement to thefibre in FIG. 1, where a number of innermost holes around the core havebeen replaced by low-index glass features. In the example shown in FIG.5, three air holes have been replaced by low-index glass features aroundthe core.

In a preferred embodiment, the low-index cladding features 52 and thefirst inner cladding features have a substantially similar size.

In another preferred embodiment n₂ is lower than n_(b).

In a further preferred embodiment, the first inner cladding features aremade of silica or doped silica, such as F-doped silica.

The here-disclosed design ideas may also be used to provide polarizationmaintaining nonlinear optical fibre. FIG. 6 shows schematically anexample of an embodiment of a fibre according to the present invention,where the number of second inner cladding features 61 is two. The fibrehas a substantially two-fold symmetry in the fibre cross-section inorder to enhance birefringence.

FIG. 7 shows the results of calculating dispersion and effective areafor a specific fibre with a design as shown schematically in FIG. 5. Thefibre has a core 51 that is formed from a Ge-doped silica rod havingn_(c) of around 1.487, and a diameter of the doped part being equal toΛ, where Λ is the centre-to-centre distance between two low-indexcladding features. The cladding background material is pure silica withn_(b) of around 1.444, and the second inner cladding features have n₂ ofaround 1.439. The first inner cladding features and the low-indexcladding features are similar and comprise air or vacuum, and they havea diameter of around 0.5Λ. Λ for the fibre is around 1.37 μm. FIG. 7shows that there is a dramatic reduction of the dispersion slope forthis fibre—as compared to the fibre of FIG. 3, while a small mode fieldarea is maintained. Also the fibre in FIG. 7 has low confinement lossdue to the relatively large size (d/Λ around 0.5) of the low-indexcladding features.

From a fabrication point of view, it is an advantage that the firstinner cladding features and the low-index cladding features have asubstantially similar size. This allows for example use of similarpreform elements to be used for the realization of the inner and outerparts of the microstructured cladding. Also it is an advantage thatholes of substantially similar size are used, since it may be moredifficult to control hole sizes accurately during fibre fabrication ifholes of different sizes are present. For example using pressure controlduring drawing of the fibre, it is an advantage that a similar pressuremay be applied to all holes for embodiments of fibres according to thepresent invention.

FIG. 8 shows the field distribution of the fundamental mode of the fibrein FIG. 7 at a wavelength of 1.55 μm. The mode is confined to the coreof the fibre and only has a limited amount of its energy distributedoutside the core.

FIG. 9 shows the influence of core size and core doping level on thedispersion properties of a fibre with a design as shown schematically inFIG. 5. The solid curve shows the dispersion for a fibre where a part ofthe core has been up-doped to provide a 2% refractive index increasecompared to pure silica. The diameter of the doped part of the core is0.5Λ. The fibre has further n₂ equal to n_(b) (no down-doping of thesecond inner cladding features; n₂ and n_(b) being equal to therefractive index of pure silica that is around 1.444 at a wavelength ofaround 1.55 μm). The long-dashed curve shows the dispersion for asimilar fibre as the fibre above, but with n2 equal to 1.439, and thesecond inner cladding features having a diameter of Λ. The short-dashedcurve shows dispersion for a fibre similar to the fibre of thelong-dashed curve, but with a core being fully doped over its diameter,Λ. Finally, the dashed-dotted curve shows the dispersion of the samefibre as in FIG. 7. This fibre is similar to the fibre of theshort-dashed curve, but with a doped core having a refractive indexbeing 3% higher than pure silica. As seen from FIG. 9, the use of secondinner cladding features provides a reduced dispersion slope. As furtherseen from FIG. 9, the dispersion characteristics may be tuned by tuningthe refractive index profile of the core. Further means for tuning thedispersion characteristics include adjusting the number, size,separation, and position of the low-index cladding features, of thefirst inner cladding doping level, as well as of the second innercladding features.

FIG. 10 shows the influence of core size and core doping level on theeffective area of the same series of fibres as in FIG. 9. Thecharacteristics of the fibres for the four different curves aredescribed for FIG. 9. FIG. 10 shows that fibres according to the presentinvention provide relatively low effective areas—corresponding torelatively high nonlinear coefficients. Hence, in combination, FIGS. 9and 10 demonstrate that embodiments according to the present inventionprovides improved nonlinear optical fibres where it is possible toobtain a relatively high nonlinear coefficient and a high control overthe dispersion and dispersion slope characteristics. In particular, thepresent invention provides an improved nonlinear fibre, having a smalleffective area (a high nonlinear coefficient) and a zero or near-zerodispersion wavelength around 1550 nm, as well as a flat dispersion (areduced dispersion slope).

Fibres according to the present invention have many degrees of freedomfor tailoring their properties for a given application. As an example ofthe flexibility in tailoring the dispersion slope of a fibre with ZDWaround 1550 nm, FIG. 11 shows the dispersion properties for a series ofembodiments of fibres with a design a shown schematically in FIG. 5 andcharacteristics as for the fibre in FIG. 7, but d/Λ for the low-indexcladding features varying from 0.44 to 0.56 and Λ varying from 1.24 μmto 1.61 μm. As seen from the figure, the dispersion slope may betailored from both negative, zero and positive values around 1550 nm byadjusting the above-mentioned parameters.

To further demonstrate the flexibility for fibres according to thepresent invention, FIG. 12 shows the dispersion properties of theembodiment of a fibre shown in FIG. 7, where Λ is varied from around1.34 μm to around 1.40 μm. FIG. 13 shows the effect on the dispersionand effective area of adjusting the refractive index of the second innercladding features for the fibre of FIG. 7. In FIG. 13, n₂ is varied from0.3% to 0.5% lower than pure silica. As seen from FIG. 13, thedispersion properties are affected to a small degree, whereas theeffective area is lowered for the decreased value of n₂.

Optical fibres according to the present invention are also of interestfor applications at other wavelengths than around 1.3 μm to 1.7 μm, suchas for applications at visible wavelengths and short near-infraredwavelengths—such as wavelengths from 400 nm to 1.3 μm, in particular forapplication at wavelengths around 800 nm and around 1064 nm. As anexample, FIG. 14 demonstrates how the dispersion slope may be decreasedfor a nonlinear fibre with ZDW around 1.06 μm using a design asschematically shown in FIG. 5 (solid curve) as compared to a fibre witha design as schematically shown in FIG. 1 (dashed curve). The dashedcurve is for a fibre comprising a doped core with n_(c)=1.472, andcladding holes having d=0.53Λ and Λ=2.05 μm, whereas the solid curveshows the dispersion for a fibre comprising a doped core withn_(c)=1.472, second inner cladding features with n₂=1.440, and low-indexcladding features having diameter 0.5Λ and Λ=1.2 μm, and first innercladding features of similar size as the low-index cladding features.Both fibres have pure silica cladding background material. As seen fromFIG. 14, the use of second inner cladding features provides means forreducing the dispersion slope. As previously discussed, such a propertymay be desired for a number of reasons, including, but not limited to,reduction of threshold power of non-linear effect and broader bandwidthof a nonlinear device.

FIG. 15 shows an exemplary preform for fabricating a fibre according tothe present invention. In the center, the preform comprises a rod 151.This rod may preferably be a silica rod doped with for example Ge havinga diameter of a few millimeters (for example 3 mm). Such doped rods maybe purchased from various commercial suppliers, e.g. FiberCore. Outsidethe core rod is placed a first ring of preform elements. These preformelements include capillary tubes 152 and solid low-index rods 153, thatwill provide the first and second inner cladding features in the finalfibre. The capillary tube may be purchased from various commercialsuppliers, e.g. Hereaus, and the low-index rods are preferably F-dopedsilica that may also be purchased from various commercial suppliers,e.g. Hereaus or ShinEtsu. Outside the first ring of preform elements isplaced a number of capillary tubes 154 that provides the outer part ofthe microstructured cladding. These capillary tubes are preferablyidentical to capillary tubes in the inner cladding. The preform elementsare placed in an overcladding tube 155 (available also from e.g.Hereaus) that provides mechanical support and holds together thepreform. The preform may further comprise various stuffing elements 156to support the capillary tubes and aid in maintaining them in a desiredposition. The various elements of the preform may be stacked in variousarrangements to yield a desired structure—as would be known to a personskilled in the art. Also other manners of realizing the preform, forexample using drilling of holes in glass rods, extrusion or sol-geltechniques may be preferred—as described in the prior art, see forexample WO 00/06506 or EP 1 172 339. Further information on fabricatingmicrostructured fibres may be found in U.S. Pat. No. 5,802,236 or WO00/49436. The preform and the preform elements may optionally be fixedby inserting the preform into a lathe where the preform is heated tomelt together all or part of the preform elements. Preferably, apressure may be applied to the capillary tubes to maintain them open.Also, a less than atmospheric pressure may be applied between thepreform elements in order to seal them together and collapseinterstitial voids in the preform.

The preform in FIG. 15 may be drawn into a cane with a diameter of a fewmillimeters, for example 4 mm, using a conventional drawing tower thatis operated at a temperature of around 1900 degrees Celsius. Preferablythe pressure in the capillary tube is controlled during drawing of thecane. Alternatively, the capillary tubes may be closed at their top endto ensure that holes do not collapse. Alternatively, the capillary tubesare kept open and a less than atmospheric pressure is applied betweenthe capillary tubes (inside the overcladding tube) to collapse tointerstitial voids between the various preform elements. The size of theholes and the closure of interstitial voids may be controlled byadjusting the pulling and/or preform feeding speed, and temperatureduring drawing—as would be known to a person skilled in the art ofproducing microstructured fibres. After fabrication of the preform cane,the cane may be further overcladded as shown in FIG. 16, where thepreform cane 161 has been overcladding by two overcladding tubes 162 and163. This new preform may be drawn into fibre using a similarconventional drawing tower. Preferably the pressure in the hole in thepreform cane is controlled and/or adjusted during fibre drawing.Preferably, a less than atmospheric pressure is applied between thepreform cane and the overcladding tube as well as between the twoovercladding tubes in order to seal together the preform cane and thetwo overcladding tubes. Naturally, a single overcladding tube or moreoverclading tubes may be used. Alternatively, the sealing of the preformcane and the overcladding tube(s) may be performed at a lathe prior tofibre drawing. The fibre is preferably drawn to an outer diameter of 125μm and one or more layers of coating as known from standard opticalfibre technology is applied.

1. An optical fibre for transmitting light of at least one predeterminedwavelength λ, the optical fibre having a longitudinal direction and across-section perpendicular thereto, the optical fibre comprising: (a) acore region comprising a core material of refractive index n_(c); and(b) a cladding region, said cladding region surrounding said core regionand comprising low-index cladding features of refractive index n₁arranged in a background cladding material of refractive index n_(b);said cladding region further comprising first inner cladding features ofrefractive index n₁ and second inner cladding features of refractiveindex n₂, where n₂ is lower than n_(c), and n₂ is higher than n₁, andsaid first inner cladding features and said second inner claddingfeatures are arranged proximal to said core region, and n₂ is lower thann_(b).
 2. The optical fibre according to claim 1, wherein a number orall of said low-index cladding features have a diameter d_(1,outer) anda centre-to-centre spacing Λ_(1,outer), and d_(1,outer) is around0.4Λ_(1,outer) or larger.
 3. The optical fibre according to claim 1,wherein a number or all of said first inner cladding features have adiameter d_(1,inner) and a centre-to-centre spacing Λ_(1,inner) andd_(1,inner) is around 0.4Λ_(1,inner) or larger.
 4. The optical fibreaccording to claim 2, wherein a number or all of said first innercladding features have a diameter d_(1,inner), and d_(1,inner) is around0.4Λ_(1,outer) or larger.
 5. The optical fibre according to the claim 2,wherein d_(1,outer) is in the range of 0.4Λ_(1,outer) to0.60Λ_(1,outer).
 6. The optical fibre according to claim 4, whereind_(1,inner) is in the range of 0.4Λ_(1,outer) to 0.60Λ_(1,outer).
 7. Theoptical fibre according to claim 2, wherein a plurality or all of saidfirst inner cladding features have a diameter d_(1,inner) and whereind_(1,outer), and d_(1,inner) are substantially similar.
 8. The opticalfibre according to claim 2, wherein a plurality or all of said firstinner cladding features have a centre-to-centre spacing Λ_(1,inner) andwherein Λ_(1,outer) and/or Λ_(1,inner) is in the range of 1.0 μm to 2.0μm.
 9. The optical fibre according to claim 2, wherein a plurality orall of said first inner cladding features have a diameter d_(1,inner)and wherein d_(1,outer) and/or d_(1,inner) is in the range of 0.4 μm to1.2 μm.
 10. The optical fibre according to claim 1, wherein said coreregion comprises silica and/or silica doped with a material to providean increase of refractive index compared to pure silica.
 11. The opticalfibre according to claim 2, wherein n_(c) is about or more than 1%higher than the refractive index of pure silica.
 12. The optical fibreaccording to claim 1, wherein n_(c) is larger than 1.444.
 13. Theoptical fibre according to claim 1, wherein said first inner claddingfeatures comprise silica or silica doped with a material to provide adecrease of refractive index compared to pure silica.
 14. The opticalfibre according to claim 2, wherein n₂ is about or more than 0.1% lowerthan the refractive index of pure silica.
 15. The optical fibreaccording to claim 1, wherein n₂ is lower than 1.444.
 16. The opticalfibre according to claim 1, wherein said low-index cladding features andsaid first inner cladding features are voids.
 17. The optical fibreaccording to claim 1, wherein said optical fibre comprises at least fiverings or periods of low-index cladding features surrounding the core.18. The optical fibre according to claim 1, wherein a majority of saidlow-index cladding features are substantially equal in size.
 19. Theoptical fibre according to claim 1, wherein said low-index claddingfeatures within a distance of at least three periods from the core aresub-stantially equal in size.
 20. The optical fibre according to claim1, wherein said core region comprises a core feature having a diameter,d_(c,feat), said core feature having a refractive index profile with amaximum refractive index being equal to n_(c).
 21. The optical fibreaccording to claim 2, wherein said core region comprises a core featurehaving a diameter, d_(c,feat), and wherein d_(c,feat) is in the range of0.2Λ_(1,outer) to 1.0Λ_(1,outer).
 22. The optical fibre according toclaim 1, wherein said optical fibre comprises a limited number of secondinner cladding features, said limited number being equal to two orthree.
 23. The optical fibre according to claim 1, wherein saidpredetermined wavelength λ is in the range of 1.3 μm to 1.7 μm.
 24. Theoptical fibre according to claim 1, wherein said predeterminedwavelength λ is in the range of 0.6 μm to 1.0 μm.
 25. The optical fibreaccording to claim 1, wherein said predetermined wavelength λ is in therange of 1.0 μm to 1.3 μm.
 26. A method of producing a microstructuredoptical fibre, the method comprising: (a) providing a preform byproviding and arranging preform elements in a predetermined structure:at least one high-index rod to form a core region of the preform, andsurrounding said high-index rod by first elements of capillary tubes andlow-index rods to form an inner cladding region proximal to said atleast one high-index rod, said capillary tubes and low-index rodsproviding first and second inner cladding features in a final fibre, andsurrounding said at least one high-index rod, said first elements ofcapillary tubes and said low-index rods by outer elements of capillarytubes providing an outer part of the micro-structured cladding; and (b)consolidating said structure; and (c) drawing said preform to an opticalfibre with pre-determined dimension under a controlled heat treatment.27. A method of guiding electromagnetic waves comprising providing anoptical fibre with a core region comprising a core material ofrefractive index n_(c) and a cladding region, said cladding regionsurrounding said core region and comprising low-index cladding featuresof refractive index n₁ arranged in a background cladding material ofrefractive index n_(b), said cladding region further comprising firstinner cladding features of refractive index n₁ and second inner claddingfeatures of refractive index n₂, where n₂ is lower than n_(c), and n₂ ishigher than n₁, and said first inner cladding features and said secondinner cladding features being arranged proximal to said core region, andwith n₂ being lower than n_(b); and feeding said optical fibre withelectromagnetic waves having wavelength from 400 nm to 1.7 μm.
 28. Anarticle comprising an optical fibre for transmitting light of at leastone predetermined wavelength λ, the optical fibre having a longitudinaldirection and a cross-section perpendicular thereto, the optical fibrecomprising: (a) a core region comprising a core material of refractiveindex n_(c); and (b) a cladding region surrounding said core region andcomprising low-index cladding features of refractive index n₁ arrangedin a background cladding material of refractive index n_(b), saidcladding region further comprising first inner cladding features ofrefractive index n₁ and second inner cladding features of refractiveindex n₂, wherein n₂ is lower than n_(c), and n₂ is higher than n₁, saidfirst inner cladding features and said second inner cladding featuresbeing arranged proximal to said core region, and n₂ being lower thann_(b).
 29. The article according to claim 28, wherein said article is acoated optical fibre or a cabled optical fibre.
 30. The articleaccording to claim 28, wherein said article is a nonlinear device. 31.The article according to claim 28, wherein said article is an opticalcommunication system, an optical fibre laser, an optical fibreamplifier, an optical fibre Raman amplifier, a dispersion compensator, adispersion and/or dispersion slope compensator, a combined dispersioncompensator and Raman amplifier, a combined dispersion slope compensatorand Raman amplifier, a combined dispersion and dispersion slopecompensator and Raman amplifier, or a super-continuum generator.
 32. Themethod as defined in claim 26, further comprising using said opticalfibre to guide electromagnetic waves by incorporating said optical fibreinto an article selected from the group consisting of an opticalcommunication system, an optical fibre laser, an optical fibreamplifier, an optical fibre Raman amplifier, a dispersion compensator, adispersion and/or dispersion slope compensator, a combined dispersioncompensator and Raman amplifier, a combined dispersion slope compensatorand Raman amplifier, a combined dispersion and dispersion slopecompensator and Raman amplifier, and a super-continuum generator; andfeeding said optical fibre with electromagnetic waves having wavelengthfrom 400 nm to 1.7 μm.
 33. A method of producing a microstructuredoptical fibre, the method comprising: providing a preform by providingand arranging preform elements in a predetermined structure: at leastone high-index rod to form a core region of the preform, and surroundingsaid high-index rod by first elements of capillary tubes and low-indexrods to form an inner cladding region proximal to said at least onehigh-index rod, said capillary tubes and low-index rods providing firstand second inner cladding features in a final fibre, and surroundingsaid at least one high-index rod, said first elements of capillary tubesand said low-index rods by outer elements of capillary tubes providingan outer part of the micro-structured cladding; and drawing said preformto a preform cane with predetermined dimension under a controlled heattreatment, followed by drawing said preform cane to an optical fibrewith predetermined dimension under a controlled heat treatment.
 34. Amethod of producing a microstructured optical fibre, the methodcomprising: providing a preform by providing and arranging preformelements in a predetermined structure: at least one high-index rod toform a core region of the preform, and surrounding said high-index rodby first elements of capillary tubes and low-index rods to form an innercladding region proximal to said at least one high-index rod, saidcapillary tubes and low-index rods providing first and second innercladding features in a final fibre, and surrounding said at least onehigh-index rod, said first elements of capillary tubes and saidlow-index rods by outer elements of capillary tubes providing an outerpart of the micro-structured cladding; and drawing said preform to anoptical fibre with predetermined dimension under a controlled heattreatment.
 35. The optical fibre according to claim 1, wherein saidoptical fibre comprises at least seven rings or periods of low-indexcladding features surrounding the core.
 36. The optical fibre accordingto claim 1, wherein said low-index cladding features within a distanceof at least five periods from the core are substantially equal in size.